15.3.2 Space Needed for Keys

MyISAM tables use B-tree indexes. You can
roughly calculate the size for the index file as
(key_length+4)/0.67, summed over all keys. This
is for the worst case when all keys are inserted in sorted order
and the table doesn't have any compressed keys.

String indexes are space compressed. If the first index part is a
string, it is also prefix compressed. Space compression makes the
index file smaller than the worst-case figure if a string column
has a lot of trailing space or is a
VARCHAR column that is not always
used to the full length. Prefix compression is used on keys that
start with a string. Prefix compression helps if there are many
strings with an identical prefix.

In MyISAM tables, you can also prefix compress
numbers by specifying the PACK_KEYS=1 table
option when you create the table. Numbers are stored with the high
byte first, so this helps when you have many integer keys that
have an identical prefix.